Philipp Podsiadlowski

Abstract:

We explore, using an exact cooling code, the thermal evolution of a neutron star undergoing episodes of intense accretion, alternated by long periods of quiescence (e.g. soft X-ray transients; SXRTs here-on). We find that the soft component of the quiescent luminosity of Aql X-1, 4U 1608-522 and of SAX J1808.4-3658 can be understood as thermal emission from a cooling neutron star with negligible neutrino emission. In the case of Cen X-4 strong neutrino emission from the inner core is necessary to explain the observation: this may indicate that the neutron star of Cen X-4 is heavier than 1.4 Mcircle dot. This study opens the possibility of using the quiescent emission of SXRTs as a tool for probing the core superfluidity in relation to the mass of the neutron star.

Abstract:

We model the evolution of binary orbits in hierarchical triple stars using a force law which includes a combination of point-mass gravity, quadrupolar distortion of the stars due to each other as well as to their intrinsic spin, a dissipative tidal friction term, and general relativity. We apply this model to the binary orbit in the present day Algol (beta Per) as well as to hypothetical young Algol models with a detached binary of two ZAMS stars. We find that although at present the semi-detached binary orbit is practically unaffected by perturbations, if the initial binary had a period of P greater than or equal to 3 days such perturbations would have shortened it to under 3 days. This sets an upper limit to the amount of angular momentum loss that the binary may have subsequently experienced, when one component became a magnetically-active red subgiant.

Abstract:

I investigate variations of the mass transfer rate in cataclysmic variables (CVs) induced by nova outbursts. The ballistic ejection of nova shells leads to a spectrum of observed transfer rates in a sample of systems with similar orbital period. I present an analytical model and show that the effect is negligible below the period gap and for CVs with high-mass white dwarfs. If the orbital period is longer than 3 hr, the mass transfer rate spectrum has an effective width in excess of an order of magnitude provided the mass accreted on the white dwarf prior to the runaway is in excess of a few 10(-4) M..

Abstract:

OA01657-415 is only the seventh eclipsing X-ray pulsar known and therefore has the potential to yield only the seventh mass of a neutron star in an X-ray binary. Here we report photometric and spectroscopic observations of candidates for the optical counterpart to the system and identify a B5III or B6V star as a possible companion to the neutron star. We measure the observational parameters of the star and suggest reasons why OAO1657-415 may be unlike other high mass X-ray binaries.

Abstract:

I present a variation on Osaki's tidal-thermal-instability model for SU UMa behaviour. I suggest that in systems with the lowest mass ratios, the angular-momentum dissipation in an eccentric disc is unable to sustain the disc on the hot side of the thermal instability. This decoupling of the tidal and thermal instabilities in low-q systems allows a better explanation of the 'echo' outbursts of WZ Sge systems and the short supercycles of ER UMa stars. The idea might also apply to the soft X-ray transients.